1. FIELD OF THE INVENTION
This invention concerns an improved method for regenerating particulate adsorbent and particularly for regenerating activated carbon adsorbent.
2. STATEMENT OF THE PRIOR ART
Activated carbon is a widely-used adsorbent for removing organic contaminants from gas streams and liquid streams, hereinafter referred to collectively as fluid streams. Activated carbon is particularly useful for removing organic contaminants from fluid streams in which the organic contaminants constitute a minor portion (less than 1%) of a fluid stream. For example: removing paint solvents from ventilation gases exhausted from paint spray operations; recovering gasoline vapors from air; removing cooking odor-causing ingredients from exhaust gases from kitchens; recovering printing ink solvents from exhaust gases of printing plants; recovering fugitive organic contaminants from the exhaust air at tank farms and transfer pumps in distribution terminals; removing organic contaminants from the exhaust gases at coating and calendering shops. Other particulate adsorbents are silica gel, activated alumina and molecular sieves.
All of these clean-up treatments involve a gas (usually air) stream containing varying amounts, but often less than one percent by volume of the organic contaminant. The contaminated gas stream is delivered through a bed of activated carbon which adsorbs the organic contaminant. The treated gas (usually air) normally can be discharged into the atmosphere or recycled as a stream containing less than the prescribed quantity of the contaminant. Many of the same types of organic materials can be removed from waste water or ground water with activated carbon.
While the expression "contaminant" is employed in this specification, there are instances in which the adsorbate is a valuable material to be recovered. The expression "contaminant" is intended to refer to the ingredient which is present in the fluid material in small quantities and is recovered on the activated carbon as the adsorbate.
Customarily the activated carbon is provided in several vessels which cycle through (a) a working stage (during which contaminants are adsorbed on the activated carbon) and (b) a regeneration stage (during which the adsorbed contaminants are removed from the activated carbon and the ability of the activated carbon to adsorb more contaminants is restored). If the service life of the activated carbon is sufficiently long, the spent activated carbon may be removed periodically for reactivation elsewhere, or perhaps discarded and replaced.
Virgin activated carbon is customarily provided as screened particles, usually 1/8 inch to 3/8 inch size, or as pellets of similar size. Activated carbon intended for use with liquids is usually smaller, e.g., 1.0 to 1.5 mm diameter. The virgin activated carbon has a large surface area per unit weight. This surface area is available for adsorbing organic contaminants. As the organic contaminants are adsorbed on the activated carbon, the remaining surface area available for further adsorption decreases and the effectiveness of the activated carbon is reduced. The spent activated carbon is regenerated or replaced when its effectiveness has reached a pre-determined minimum acceptable value. The minimum acceptable value will be determined by the requirements of the installation, e.g., the allowable contaminant content of the discharged gas.
There are several commercial regeneration procedures. In some installations the spent activated carbon is removed from the vessel and is replaced with virgin activated carbon or with off-site regenerated activated carbon or with a mixture of both. Replacement with virgin activated carbon is costly but may be justified if substantially total contaminant removal is required. Movement of the activated carbon off-site results in transportation costs, labor costs and particle abrasion and degradation, producing fines which must be screened from the regenerated activated carbon. Regeneration off-site is usually accomplished by heating the spent activated carbon in a furnace or kiln, with steam being introduced to create a suitable atmosphere. The regeneration gases burn some of the adsorbed contaminants, and also burn some of the activated carbon with the result that there is less activated carbon and, more importantly, the residual activated carbon commonly has a lower adsorption capacity and is a less efficient adsorbent than virgin activated carbon, and may be undesirably soft and dusty. There are also known in-situ regeneration procedures using steam and/or hot gases to devolatilize adsorbate.
Regeneration of activated carbon by means of vacuum procedures has been ineffective because the adsorbent volatilization requires thermal energy with the result that the carbon is chilled before significant desorption is achieved. The chilling effect of vacuum desorption cannot be offset because the vacuum environment is known to be an efficient thermal insulator; hence heat energy cannot be added to the chilled carbon during vacuum desorption.
Regeneration of activated carbon by means of vacuum procedures has been ineffective because the adsorbent volatilization requires thermal energy with the result that the carbon is chilled before significant desorption is achieved. The chilling effect of vacuum desorption cannot be offset because the vacuum environment is known to be an efficient thermal insulator; hence heat energy cannot be added to the chilled carbon during vacuum desorption.
According to the preferred embodiment of this invention, the regeneration is carried out in situ, i.e., in the same vessel, without removing the spent activated carbon. This preferred embodiment is particularly useful when the duty cycle of the units is short, e.g., 30 minutes to several weeks.
In general the regenerated activated carbon never achieves the adsorbing capacity and effectiveness of the virgin activated carbon because there is some residual adsorbate which resists separation from the activated carbon regardless of the regeneration procedure. This is especially common with in situ steam-regenerated activated carbon, where practical considerations usually demand that steaming the activated carbon be curtailed before the residual adsorbed substances (those in the highest adsorption energy portions of the adsorbent structure) are removed.
In many activated carbon treatment processes, the regenerated activated carbon has its effectiveness reduced to such a level that the system cannot satisfy demanding contaminant removal requirements in subsequent cycles, and thus regenerated activated carbon cannot meet the requirement at all or can meet the requirement for only uneconomically brief cycles.